Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

There are lots of subclasses to galaxies, usually defined by some special characteristic that sets one apart, like an oddly shaped core or a big puffy disk. Today, astronomers announced a wholly new kind of subclass: green bean galaxies. It’s a funny name, but it fits: These galaxies are, in fact, green!

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The first one found, called J224024.1−092748 or J2240 for short, is not exactly a close neighbor, lying at a distance from Earth of a staggering 3.7 billion light years. That is a long way off, which turns out to be one of the keys to understanding its nature. As we look away in distance, we see further back in time because it takes time for the light to reach us. We’re seeing this galaxy as it was when it was substantially younger, so whatever is making it green may be related.

Most galaxies are red or blue or some combination of the two. The light we see comes mostly from stars. Roughly speaking, young, massive stars are hot and blue, while older stars tend to be red. But there are no green stars! Due to the way hot objects glow, combined with how our eyes see light, it’s not possible for a star to glow green. So why is J2240 so verdant?

It has gas. I’m not kidding! Giant clouds of gas glow at very specific colors depending on how dense they are, how hot they are, and what they’re made of. Warm, thin hydrogen emits red light, for example. Oxygen, especially when it’s hot and extremely rarefied, glows a ghostly green … and that’s what we’re seeing here with this distant galaxy. J2240 is filled with gas that has enough oxygen to color the whole galaxy green. Now, I’m not talking about a breathable atmosphere here; the gas is so thin it’s almost like a laboratory vacuum. But over thousands of light years, even that ethereal soup mist adds up. In this case, all that oxygen is glowing so fiercely that by itself, it’s emitting energy equal to to ten billion times what the Sun puts out! That’s astonishing, and way brighter than any other such glowing gas ever seen.

Lots of galaxies have gas clouds, though. The Milky Way does, for example. Why aren’t we green? What’s different about J2240?

It appears that the gas content of the galaxy is higher than most, but still, something needs to light up all that gas. The best engine we know of that can do that is a black hole. And not just any black hole, but one we’ve caught in the act of starving.

Every big galaxy has a supermassive black hole in its core, millions or billions of times the mass of the Sun. If matter falls in, the material can form a flattened disk called an accretion disk. This stuff swirls madly around the black hole, with the inner edge moving very nearly at the speed of light. Friction and other forces inside the disk are hellishly powerful, enough to heat the material up to millions of degrees. Matter at this temperature glows madly, and accretion disks in galactic cores can outshine all the stars in the galaxy combined! We call these objects active galaxies.

This is almost certainly what excited all that gas in the J2240. But if that’s the case, why don’t we see the incredibly bright light from the disk in the galaxy’s center? Astronomers think that’s because the black hole was feeding at some indeterminate time in the past, but has since shut off. Perhaps, long ago, J2240 collided with another galaxy, dumping material into the hole, or maybe some other disturbance fed it material which then formed the accretion disk. But after time, the supply cut off, and the last of the matter fell in. The disk disappeared, the energy source shut off, and the brilliant heart of the galaxy dimmed. The gas outside, in the greater galaxy, can continue to glow for quite some time, fading all the while, but it is still bright enough for us to spot billions of light years away.

And that distance is important. We see lots of active galaxies when we look into the more distant Universe, when it was young. They were more common then. They aren’t terribly common now, so at some point in the past they shut off. Three to five billion years ago is a reasonable time frame for that to have happened, fitting J2240’s distance. Moreover, this stage where the black hole starves is very short compared to the life of the galaxy, so seeing a black hole as it wanes would be rare. Once astronomers found J2240 they sifted through a database containing the colors of billions of galaxies and found 16 more. That’s it. That counts as rare to me.

All 17 known green bean galaxies in the Universe. J2240 is listed as number 16.

Image credit: CFHT/ESO/M. Schirmer

So these galaxies represent a very precious specimen in the astronomical bestiary: a brief, shining moment when the dragon at the heart of a galaxy stops roaring, but the energy of its outburst lingers. The green glow is literally the echo of light from the dying black hole.

And I’ll note that black hole isn’t really dead. It’s dormant. It’s still there, still massive, still possessing fierce gravity. If another gas cloud drops in or another galaxy passes too close, the beast could awaken again. We do see some active galaxies near us, and we have our own supermassive black hole as well. We don’t think it will ever get as bright as the ones in green bean galaxies, but it does sometimes decide to have a snack or two. I don’t think it would present any danger to us, necessarily, and the opportunity to study one of these monsters up close would be a tremendous chance for astronomers to understand better what lies at our own Milky Way’s heart.

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